What is the mass of an electron as a fraction of the mass of a proton or neutron?
This question is asking us to express the mass of an electron as a fraction. The word “of” that follows the word “fraction” in the question indicates that the mass of one electron is less than the mass of one proton or one neutron. We are not being asked to give an experimentally observed or measured mass of an electron. What we need to do to solve this problem is determine how the mass of an electron compares to the mass of a proton or neutron.
When the mass of a particle, such as an electron or an atom, is compared to the mass of another particle, it’s called the relative mass. The three main types of subatomic particles found inside atoms are electrons, protons, and neutrons. The relative charges of these subatomic particles are negative one, positive one, and zero, respectively. The observed masses of protons and neutrons are often reported using the units kilograms. However, because the masses of subatomic particles are so small, chemists use the unified atomic mass unit, or u, to compare subatomic particles and atoms and to perform calculations. By definition, one unified atomic mass unit is equal to one twelfth of the mass of a single carbon-12 atom. The approximate mass of a single proton or neutron is about 1.01 unified atomic mass units. In contrast, the approximate mass of an electron is about 0.00055 unified atomic mass units.
Now, let’s use these approximate masses to determine just how much more massive a proton is than an electron. We can do this by dividing the mass of one proton by the mass of one electron. The result of dividing 1.01 u by 0.00055 u is about 1840. The mass of a proton is essentially the same as the mass of a neutron, roughly one unified atomic mass unit. So chemists assign the relative mass of a proton and neutron to be one. However, the mass of about 1840 electrons would have the same mass as a single proton or neutron. So we can write the relative mass of the electron as one over 1840. The mass of an electron expressed as a fraction of the mass of a proton or neutron is one over 1840.